1. Field of the Invention
The present invention relates to a CDMA cellular system.
2. Description of the Related Art
In a code division multiple access (CDMA) cellular system, a plurality of terminals such as mobile phones can be accommodated in a service area by dividing the service area into cells, and by using different codes in each of frequency bands.
FIG. 1 shows a conventional general system configuration of the CDMA cellular system. Referring to FIG. 1, the CDMA cellular system is composed of base station control apparatuses 100-1 and 100-2, radio base stations 200-1, 200-2, 200-3, 200-4, 200-5, and 200-6, radio terminals 300-1 and 300-2, a cellular system switching apparatus 400, and a network management system apparatus 420.
The cellular system switching apparatus 400 is connected in wires with a general network 500 and the base station control apparatuses 100-1 and 100-2 arranged for the respective service areas, each of which is composed of a plurality of cells. The base station control apparatus 100-1 is connected in wires with the radio base stations 200-1, 200-2 and 200-3 which are arranged for the respective cells, and the base station control apparatus 100-2 is connected in wires with the radio base stations 200-4, 200-5 and 200-6 which are arranged for the respective cells.
The radio terminals 300-1 and 300-2 are connected in radio with the radio base stations corresponding to the cells where the radio terminals 300-1 and 300-2 exist, respectively. Also, the radio terminals 300-1 and 300-2 can communicate with each other through the base station control apparatuses 100-1 and 100-2 and the cellular system switching apparatus 400. Also, the radio terminals 300-1 and 300-2 can communicate with a general subscriber telephone 600 which is connected with the general network 500.
In this case, in the conventional CDMA cellular system, the radio channel to be used is selected and allocated between the radio base station and the radio terminal independently for every base station control apparatus. For this reason, there is a case that one radio channel allocated by the base station control apparatus 100-1 is same as that allocated by the base station control apparatus 100-2. In this case, if hand-over is carried out from the base station control apparatus 100-1 to the base station control apparatus 100-2, interference occurs between the radio terminals 300-1 and 300-2. Therefore, it is necessary that the radio channel allocated by the base station control apparatus 100-1 and the radio channel allocated by the base station control apparatus 100-2 are different from each other.
For the purpose of solving the above problem, a method is proposed in which when a call is issued, the base station control apparatus communicate with another base station control apparatus to determine channels allocated already by the other base station control apparatus, and allocates a channel not yet allocated for the call. However, in this method, it is necessary that the base station control apparatus and the other base station control apparatus communicate with each other every time a call is issued. Therefore, it results in increasing the load of the base station control apparatus and other base station control apparatus.
In conjunction with a method of allocating a communication band is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 6-334594). In the method of this conventional example, non-combination groups (super cells) of cells are formed to minimize interference in each group. Specific channels of communication bands are allocated in common to the groups, respectively. The communication bands of the specific channels are dynamically allocated to cells in each group in accordance with a user request from each cell.
Also, a mobile communication system is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 9-200838. Te mobile communication system of this conventional example is composed of a mobile station, a plurality of base stations, a plurality of control stations and an integrating station. The base station provides a service for each of cells obtained by dividing a service area. The control station manages radio channels used by the base stations without recognition of an arrangement of the base stations belonging to the control station and controls the base stations. The integrating station recognizes an arrangement of the control stations and controls the control stations the radio channels used in the control stations and cell relation of cells serviced by the control stations.
Also, a communication channel selecting method is disclosed in Japanese Laid Open Patent Application (JP-P 2000-152322). In a digital cordless telephone of this conventional example, a digital cordless telephone and a plurality of radio connection apparatuses are connected with a main apparatus which accommodates lines. A plurality of communication channels are grouped into a plurality of groups. The connection apparatus searches even-numbered (odd-numbered) communication channels of a corresponding group in order for an unused channel and searches odd-numbered (even-numbered) communication channels of the corresponding group for the unused channel when the unused channel is not found in the above process.
Also, a method of allocating communication frequency band is disclosed in Japanese Laid Open Patent Application (JP-P2000-59388A). In the method of this conventional example, a terminal apparatus is physically connected to a network connection apparatus to form a radio communication apparatus. A specific frequency band is allocated to the network connection apparatus in a network based on a serial number of the network communication apparatus. The radio communication apparatuses are connected by a cable to share data of the channels.
Also, a cordless telephone apparatus is disclosed in Japanese Laid Open Patent Application (JP-P2001-78251A). The cordless telephone apparatus of this conventional example is composed of a grouping section which groups channels based on an ID code or a PN series. A changing section switches the ID code or the PN series. A storage section stores unused channels in a reception scanning. A communication channel is determined based on the unused channels before the channel group is switched, when a link formation is started before the scanning is carried out by switching the channel group.